A variety of implantable medical devices for delivering a therapy and/or monitoring a physiological condition have been clinically implanted or proposed for clinical implantation in patients. Implantable medical devices may deliver electrical stimulation or fluid therapy to, and/or monitor conditions associated with, the heart, muscle, nerve, brain, stomach or other organs or tissue.
Some implantable medical devices, such as cardiac pacemakers or implantable cardioverter-defibrillators, provide therapeutic electrical stimulation to the heart. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, an implantable medical device may sense intrinsic depolarizations of the heart, and control delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation.
Patients with heart failure are often treated with cardiac resynchronization therapy (CRT). CRT is a form of cardiac pacing. In some examples, CRT involves delivery of pacing pulses to both ventricles to synchronize their contraction. In other examples, CRT involves delivery of pacing pulses to one ventricle, such as the left ventricle, to synchronize its contraction with that of the right.
Some implantable medical devices, such as implantable pacemakers, or pacemaker-cardioverter-defibrillators, have been used for long term monitoring of heart failure patients. In some cases, such implantable medical devices monitor for an early indication of a heart failure decompensation event based on one or more physiological parameters of the patient. Example physiological parameters for monitoring heart failure include thoracic fluid accumulation, edema, various cardiac or vascular blood pressures, various hemodynamic parameters, nighttime heart rate, and respiration.